The invention relates to a device for interferometric vibration measurement on an object and to a method for such measurement.
It is known to measure vibrations on an object contactlessly using an interferometer, for example a laser Doppler interferometer. A typical interferometer of that kind comprises a radiation source, a beam splitter and a detector.
In such an arrangement, an original beam generated by the radiation source is divided by the beam splitter into a measuring beam and a reference beam. The measuring beam is guided onto a measuring point on the object and the measuring beam at least partly reflected by the object is superimposed with the reference beam on a detection area of the detector, so that using the detector, a superimposition or interference signal between measuring and reference beams can be measured.
The frequency of the measuring beam is affected by the movement or vibration of the object surface, so that on the basis of the superimposition signal of the measuring and reference beams, conclusions can be drawn as to the movement of the object, especially the vibration frequency of the object surface.
In such an arrangement, the radiation source must generate monochromatic radiation or at least radiation having a sufficiently large coherence length. Lasers are typically used as radiation sources.
Furthermore, it is known to arrange in the beam path of the measuring beam a focusing device by which the measuring beam is focussed onto the measuring point on the object.
In interferometric vibration measurement it is customary to use a laser-generated measuring beam having a wavelength in the visible range, that is to say in a wavelength range of from 380 nm to 750 nm, which in principle involves risk of injury, especially to the human eye. Increased safety precautions are therefore necessary, or a low-power laser must be used, for example a class 1 laser having an output of less than 25 μW. This is disadvantageous in terms of limitation of the range of application, the costs arising as a result of the safety precautions and/or deterioration in the quality of the measurement signals (especially the signal-to-noise ratio) on account of the low laser power and accordingly the low intensity of the measuring beam.